Fix up final FPU-registers for FreeBSD x86_64.

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                ,        ,
               /(        )`
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               /- _  `-/  '
              (/\/ \ \   /\
              / /   | `    \
              O O   ) /    |
              `-^--'`<     '
             (_.)  _  )   /
              `.___/`    /
                `-----' /
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                |        |
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This closes https://github.com/dotnet/coreclr/issues/594#issuecomment-93995183

Author: josteink

src/pal/src/include/pal/context.h

@@ -137,11 +137,7 @@ typedef ucontext_t native_context_t;
 #define FPREG_DataSelector(uc)  *((WORD*) &(FPSTATE(uc)->sv_env.en_rdp) + 2)
 
 #define FPREG_Xmm(uc, index)    *(M128A*) &(FPSTATE(uc)->sv_xmm[index])
-
-// TODO: port this register to FreeBSD.
-// #define FPREG_St(uc, index)     *(M128A*)&((uc)->mc_fpstate[index])
-// something like this?
-// #define FPREG_St(uc, index)     *(M128A*)&(FPSTATE(uc)->sv_xstate.sx_ymm[index])
+#define FPREG_St(uc, index)     *(M128A*) &(FPSTATE(uc)->sv_fp[index].fp_acc)
 
 #else // BIT64