[compiler-rt][sanitizers] Fix GetPcSpBp determination of SP on 32-bit Solaris/x86

A dozen 32-bit `AddressSanitizer` testcases FAIL on the latest beta of Solaris 11.4/x86, e.g.
`AddressSanitizer-i386-sunos :: TestCases/null_deref.cpp` produces

  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==29274==ERROR: AddressSanitizer: stack-overflow on address 0x00000028 (pc 0x08135efd bp 0xfeffdfd8 sp 0x00000000 T0)
      #0 0x8135efd in NullDeref(int*) /vol/llvm/src/llvm-project/dist/compiler-rt/test/asan/TestCases/null_deref.cpp:15:10
      #1 0x8135ea6 in main /vol/llvm/src/llvm-project/dist/compiler-rt/test/asan/TestCases/null_deref.cpp:21:3
      #2 0x8084b85 in _start (null_deref.cpp.tmp+0x8084b85)

   SUMMARY: AddressSanitizer: stack-overflow /vol/llvm/src/llvm-project/dist/compiler-rt/test/asan/TestCases/null_deref.cpp:15:10 in NullDeref(int*)
  ==29274==ABORTING

instead of the expected

  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==29276==ERROR: AddressSanitizer: SEGV on unknown address 0x00000028 (pc 0x08135f1f bp 0xfeffdf48 sp 0xfeffdf40 T0)
  ==29276==The signal is caused by a WRITE memory access.
  ==29276==Hint: address points to the zero page.
      #0 0x8135f1f in NullDeref(int*) /vol/llvm/src/llvm-project/local/compiler-rt/test/asan/TestCases/null_deref.cpp:15:10
      #1 0x8135efa in main /vol/llvm/src/llvm-project/local/compiler-rt/test/asan/TestCases/null_deref.cpp:21:3
      #2 0x8084be5 in _start (null_deref.cpp.tmp+0x8084be5)

  AddressSanitizer can not provide additional info.
   SUMMARY: AddressSanitizer: SEGV /vol/llvm/src/llvm-project/local/compiler-rt/test/asan/TestCases/null_deref.cpp:15:10 in NullDeref(int*)
  ==29276==ABORTING

I managed to trace this to a change in `<sys/regset.h>`: previously the header would
primarily define the short register indices (like `UESP`). While they are required by the
i386 psABI, they are only required in `<ucontext.h>` and could previously leak into
unsuspecting user code, polluting the namespace and requiring elaborate workarounds
like that in `llvm/include/llvm/Support/Solaris/sys/regset.h`. The change fixed that by restricting
the definition of the short forms appropriately, at the same time defining all `REG_` prefixed
forms for compatiblity with other systems.  This exposed a bug in `compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp`, however:
Previously, the index for the user stack pointer would be hardcoded if `REG_ESP`
wasn't defined. Now with that definition present, it turned out that `REG_ESP` was the wrong index to use: the previous value 17 (and `REG_SP`) corresponds to `REG_UESP`
instead.

With that change, the failures are all gone.

Tested on `amd-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D83664