This filename is output in the emitted asm files when
ENABLE_RR_EMIT_ASM_FILE is enabled, but on Windows, it was always
"<unknown>". Fixed this by splitting out the name by backslash as well.

Bug: b/174358505
Change-Id: Ie5386fa50fd8a6aa43a48bc6fa5fcf987936aacf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51750
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>