arm: asm.h - change GOT_INIT to take the normal asm label

Don't pass the label to use for the "current address" as the macro
parameter. It obscures its purpose and makes the use of local labels
extremely unobvious, so you are forced to invent a name that is mostly
useless otherwise:

GOT_INIT(r3, .Lgot.execsp_start, .Lpc.execsp_start)
GOT_INITSYM(.Lgot.execsp_start, .Lpc.execsp_start)
vs.
0: GOT_INIT(r3, .Lgot.execsp_start)
GOT_INITSYM(.Lgot.execsp_start, 0b)

Same object code is generated.

arm: cerror.S - use GOT_SYM

Use GOT_SYM instead of open-coding it. It's defined in asm.h but
doesn't seem to be used anywhere, so perk it up.

While here, be consistent about declaring errno and use that as a
chance to provide a comment why #ifdef _REENTRANT is needed here.

arm: cerror.S - factor out code to return -1

Do not repeat verbose ifdefed code that sets (long long){ r0, r1 } to
-1LL that indicates an error return from a syscall. Define it once as
a macro and use it. Fix the code to actually be compilable as thumb
("mov" vs "movs").

Modulo one fixed "mov" vs "movs" the same object code is generated for
all 8 permutations of (arm/thumb x non/reentrant x absolute/pic).

arm/asm.h: revert PCREL_GET &c in previous (ok riastradh)

This change needs to be redone and re-verified. I have detailed some
issues in the PR, and it also turns out that some of the changes were
in the !_REENTRANT branch of #ifdef and thus not checked at all.
riastradh asked to back out the changes for now.

PR lib/59391: unnecessary __PIC__ conditionals clutter .S files

arm: Nix most __PIC__ conditionals in .S files.

New macros PCREL_GET(rN,label,pclabel) and PCREL_SYM(label,pclabel)
to enable this by the pattern:

ldr rN, label
#ifdef __PIC__
pclabel:
add rN, rN, pc
#endif
...
label:
#ifdef __PIC__
.word (label - (pclabel + 2*sizeof(instruction)))
#else
.word label
#endif

(sizeof(instruction) = 2 for thumb, 4 for non-thumb.)

No binary change in libc with MKPIE=no (i.e., testing both for
changes to the PIC build and changes to the non-PIC build).

In principle, assembly routines could improve instruction scheduling
by splitting up the ldr and add instructions. But in practice, the
maintenance and auditing burden likely makes this worthwhile by
improving legibility vs a tangle of in-line #ifdefs.

brk.S does things a little differently making it harder to unify.
Could redo it but I don't want to make changes without testing them
first.

PR lib/59391: unnecessary __PIC__ conditionals clutter .S files

branches: 1.13.38;
Use PLT_SYM.

Pass PICFLAGS down to cc-as-as and use __PIC__ to decide if it is small
vs big PIC mode. Retire -DPIC and -DBIGPIC.

only emit cfi if __ARM_EABI__ && __UNWIND_TABLES__

Use less got, add END(), make thumb tolerant.

CERROR is now hidden so no need to call it via the PLT

branches: 1.8.2;
Make __cerror protected to restore ABI leak in older libc versions,
where e.g. librt referenced the copy from libc. This still allows libc
to use PC-relative jumps.

branches: 1.7.6; 1.7.8;
Change __cerror to be marked hidden and change callers to avoid the PLT.

branches: 1.6.16;
Use RET and RETc for returning.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

NetBSD/acorn26 has been using APCS-32 for years, so unifdef -U__APCS_26__.

Local label fixup.

Changes needed for ARM ELF shared library support.

branches: 1.1.2;
Create shared ARM source for libc, and move arm26 port over to using it.
The code is (almost) a direct copy of the current arm26 sources.
It's identical to the current arm32 sources, with the following exceptions:
- _C_LABEL is used on references to C labels from assembler.
- Function returns in assembler have APCS-26 versions in #ifdef __APCS_26__.
- It uses SoftFloat 2a rather than SoftFloat 1a.

The first two of these should be inconsequential. I believe that SoftFloat 2a
should work on arm32 and be backward-compatible with existing code, but this
is not obviously true. For now, arm32 remains using its own bits of libc.