[lld/mac] Parallelize code signature computation

According to ministat, this is a small but measurable speedup
(using the repro in PR56121):

    N           Min           Max        Median           Avg        Stddev
x  10     3.7439518     3.7783802     3.7730219     3.7655502   0.012375226
+  10     3.6149218      3.692198     3.6519327     3.6502951   0.025905601
Difference at 95.0% confidence
	-0.115255 +/- 0.0190746
	-3.06078% +/- 0.506554%
	(Student's t, pooled s = 0.0203008)

(Without 858e8b17f7, this change here to use parallelFor is an 18% speedup,
and doing 858e8b17f7 on top of this change is just a 2.55% +/- 0.58% win.
Doing both results in a total speedup of 20.85% +/- 0.44%.)

Differential Revision: https://reviews.llvm.org/D128298

lld/MachO/SyntheticSections.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LEB128.h"
+#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
 
 #if defined(__APPLE__)
@@ -1247,11 +1248,11 @@ void CodeSignatureSection::writeHashes(uint8_t *buf) const {
   // NOTE: Changes to this functionality should be repeated in llvm-objcopy's
   // MachOWriter::writeSignatureData.
   uint8_t *hashes = buf + fileOff + allHeadersSize;
-  for (uint64_t i = 0; i < getBlockCount(); ++i) {
+  parallelFor(0, getBlockCount(), [&](size_t i) {
     sha256(buf + i * blockSize,
            std::min(static_cast<size_t>(fileOff - i * blockSize), blockSize),
            hashes + i * hashSize);
-  }
+  });
 #if defined(__APPLE__)
   // This is macOS-specific work-around and makes no sense for any
   // other host OS. See https://openradar.appspot.com/FB8914231