Add missing define on Solaris.

XXX file upstream bug

--- mozjs-24.2.0/js/src/jit/AsmJSSignalHandlers.cpp	2013-10-29 13:40:19.000000000 -0700
+++ mozjs-24.2.0.new/js/src/jit/AsmJSSignalHandlers.cpp	2015-06-27 18:27:41.867039386 -0700
@@ -60,6 +60,7 @@
 # if defined(__linux__)
 #  define XMM_sig(p,i) ((p)->uc_mcontext.fpregs->_xmm[i])
 # else
+#  define REG_EIP EIP
 #  define XMM_sig(p,i) ((p)->uc_mcontext.fpregs.fp_reg_set.fpchip_state.xmm[i])
 # endif
 # define EIP_sig(p) ((p)->uc_mcontext.gregs[REG_EIP])