Defining ENABLE_RR_EMIT_ASM_FILE will make it so that the LLVM backend
outputs a unique asm file per routine that is generated. This file is
further processed and updated so that each instruction is prefixed with
the final resolved memory location.

This is useful, for instance, when we get JIT callstacks and can't
easily figure out which code it maps to. Furthermore, when this feature
is coupled with ENABLE_RR_DEBUG_INFO, the emitted asm includes source
location (file and line) information, making it easy to correlate the
asm to the Reactor code.

For example, running ReactorUnitTests.Sample with both
ENABLE_RR_EMIT_ASM_FILE and ENABLE_RR_DEBUG_INFO enabled generates a
file named swiftshader_jit_llvm_0000_ReactorUnitTests_Sample.asm, with
partial output like so:

```
	.file	2 "C:\\src\\SwiftShader2\\tests\\ReactorUnitTests\\ReactorUnitTests.cpp\\<unknown>"
	.loc	2 53 0 prologue_end     # <unknown>:53:0
[0x2B9D3358004] 	mov	qword ptr [rsp + 64], rcx # encoding: [0x48,0x89,0x4c,0x24,0x40]
[0x2B9D3358009] 	mov	qword ptr [rsp + 72], rcx # encoding: [0x48,0x89,0x4c,0x24,0x48]
	.loc	2 54 0                  # <unknown>:54:0
[0x2B9D335800E] 	mov	eax, dword ptr [rcx - 4] # encoding: [0x8b,0x41,0xfc]
[0x2B9D3358011] 	mov	dword ptr [rsp + 8], eax # encoding: [0x89,0x44,0x24,0x08]
[0x2B9D3358015] 	mov	dword ptr [rsp + 16], eax # encoding: [0x89,0x44,0x24,0x10]
	.loc	2 55 0                  # <unknown>:55:0
[0x2B9D3358019] 	mov	dword ptr [rsp + 12], edx # encoding: [0x89,0x54,0x24,0x0c]
[0x2B9D335801D] 	mov	dword ptr [rsp + 20], edx # encoding: [0x89,0x54,0x24,0x14]
```

The "53", "54", and "55" are the line numbers of the respective Reactor
code in ReactorUnitTests.cpp.

CMake: enable REACTOR_EMIT_ASM_FILE to enable this feature.

Bug: b/174358505
Change-Id: I613a25fe0354a1343c49cb399875e82d5e806e29
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50750
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>