[NFCI][bolt][test] Enable AT&T syntax explicitly

Files using the Intel syntax typically explicitly specify it.
Do the same for the few relevant files for AT&T.

This enables building LLVM with `-mllvm -x86-asm-syntax=intel` in one's
Clang config files (i.e. a global preference for Intel syntax).

Author: tambry

bolt/test/runtime/X86/Inputs/basic-instrumentation.s

@@ -1,3 +1,4 @@
+  .att_syntax
   .globl main
   .type main, %function
 main:

bolt/test/runtime/X86/Inputs/internalcall.S

@@ -1,6 +1,7 @@
 // This is reduced test case for BOLT containing an internal call based on
 // GetCoreDump (from google core dumper).
 
+  .att_syntax
   .text
   .globl getCallback
   .type getCallback, %function

bolt/test/runtime/X86/Inputs/pie-exceptions-failed-split.s

@@ -49,6 +49,7 @@
 #			return !throw_test(argc, argv);
 #		}
 
+	.att_syntax
 	.text
 	.file	"exceptions-failed-split.cpp"
 	.globl	_Z3bari                         # -- Begin function _Z3bari

bolt/test/runtime/X86/exceptions-lpstart-zero.s

@@ -21,6 +21,7 @@
 # The exception table was modified with udata4 LPStartEncoding and sdata4
 # CallSiteEncoding.
 
+	.att_syntax
 	.text
 	.globl	main                            # -- Begin function main
 	.p2align	4, 0x90