fix: compilation error on Windows on Arm

Enums can be unsigned or signed. This change corrects an unsigned/signed
comparison error by making sure this enum is always unsigned.

Bug: 1141421
Change-Id: Ic3576497c77fd6e3391e6d42a4653c699f337a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493201
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#820057}

base/immediate_crash_unittest.cc

@@ -62,7 +62,7 @@ using Instruction = uint32_t;
 // Use an enum here rather than separate constexpr vars because otherwise some
 // of the vars will end up unused on each platform, upsetting
 // -Wunused-const-variable.
-enum {
+enum : Instruction {
   // There are multiple valid encodings of return (which is really a special
   // form of branch). This is the one clang seems to use:
   kRet = 0xd65f03c0,