Add new form of ldr/str (immediate) to ARM integrated assembler.

tests_lit/assembler/arm32/global-load-store.ll

 ; TODO(kschimpf): Show that we can handle global variable loads/stores.
 
 ; REQUIRES: allow_dump
 
 ; Compile using standalone assembler.
 ; RUN: %p2i --filetype=asm -i %s --target=arm32 --args -O2 \
 ; RUN:   | FileCheck %s --check-prefix=ASM
 
 ; Show bytes in assembled standalone code.
 ; RUN: %p2i --filetype=asm -i %s --target=arm32 --assemble --disassemble \
@@ skipping 37 matching lines @@
 ; DIS-LABEL:00000000 <load>:
 ; DIS-NEXT:   0:   e3000000
 ; DIS-NEXT:   4:   e3400000
 ; DIS-NEXT:   8:   e5900000
 ; DIS-NEXT:   c:   e12fff1e
 
 ; IASM-LABEL:load:
 ; IASM-NEXT: .Lload$__0:
 ; IASM-NEXT:    movw    r0, #:lower16:global1
 ; IASM-NEXT:    movt    r0, #:upper16:global1
-; IASM-NEXT:    ldr     r0, [r0]
+; IASM-NEXT:    .byte 0x0
+; IASM-NEXT:    .byte 0x0
+; IASM-NEXT:    .byte 0x90
+; IASM-NEXT:    .byte 0xe5
 ; IASM-NEXT:    .byte 0x1e
 ; IASM-NEXT:    .byte 0xff
 ; IASM-NEXT:    .byte 0x2f
 ; IASM-NEXT:    .byte 0xe1
 
 define internal void @store(i32 %v) {
   %addr = bitcast [4 x i8]* @global1 to i32*
   store i32 %v, i32* %addr, align 1
   ret void
 }
 
 ; ASM-LABEL:store:
 ; ASM-NEXT: .Lstore$__0:
 ; ASM-NEXT:     movw    r1, #:lower16:global1
 ; ASM-NEXT:     movt    r1, #:upper16:global1
 ; ASM-NEXT:     str     r0, [r1]
 ; ASM-NEXT:     bx      lr
 
 ; DIS-LABEL:00000010 <store>:
 ; DIS-NEXT:  10:   e3001000
 ; DIS-NEXT:  14:   e3401000
 ; DIS-NEXT:  18:   e5810000
 ; DIS-NEXT:  1c:   e12fff1e
 
 ; IASM-LABEL:store:
 ; IASM-NEXT: .Lstore$__0:
 ; IASM-NEXT:    movw    r1, #:lower16:global1
 ; IASM-NEXT:    movt    r1, #:upper16:global1
-; IASM-NEXT:    str     r0, [r1]
+; IASM-NEXT:    .byte 0x0
+; IASM-NEXT:    .byte 0x0
+; IASM-NEXT:    .byte 0x81
+; IASM-NEXT:    .byte 0xe5
 ; IASM-NEXT:    .byte 0x1e
 ; IASM-NEXT:    .byte 0xff
 ; IASM-NEXT:    .byte 0x2f
 ; IASM-NEXT:    .byte 0xe1