use GLOBAL() macro when loading constant

Clear linker error when building with gcc 6:
relocation R_X86_64_32 against `.rodata' can not be used when making a
shared object; recompile with -fPIC

BUG=aomedia:102

Change-Id: I6c06de1e9dac1c044a4b07125abcaba0943a29b6

aom_dsp/x86/quantize_ssse3_x86_64.asm

diff --git a/aom_dsp/x86/quantize_ssse3_x86_64.asm b/aom_dsp/x86/quantize_ssse3_x86_64.asm
index 7cf4bfa..e2c1ebb 100644
--- a/aom_dsp/x86/quantize_ssse3_x86_64.asm
+++ b/aom_dsp/x86/quantize_ssse3_x86_64.asm
@@ -45,7 +45,7 @@
   psrlw                           m1, 1                    ; m1 = (m1 + 1) / 2
 %endif
   mova                            m3, [r2q]                ; m3 = dequant
-  psubw                           m0, [pw_1]
+  psubw                           m0, [GLOBAL(pw_1)]
   mov                             r2, shiftmp
   mov                             r3, qcoeffmp
   mova                            m4, [r2]                 ; m4 = shift