[PPC] Make assembler.cpp compile over 30 times faster

Clang was taking around 30 seconds to compile assembler.cpp on my
machine, with most of its time spent in code generation for the global
initializer for `lookup`.

Changing the map key from `std::string` to `std::string_view` drops
compile time to under a second. This is because the compiler no longer
has to code gen the allocation, initialization, and moves of 2,100
`std::string`s. `std::string_view` is effectively free to initialize in
comparison.

While I was here I made the map `static const`. It's not required for
the build time improvement, but it is more correct and helped me check
that no-one is mutating the map.

Author: bdash

arch/powerpc/assembler.cpp

@@ -43,7 +43,7 @@ struct info {
 	uint32_t mask; /* which bits to mutate */
 };
 
-map<string, info> lookup = {
+static const map<string_view, info> lookup = {
 {               "tdi NUM , GPR , NUM",{0x0800000A,0x03FFFFFF}}, // 000010xxxxxxxxxxxxxxxxxxxxxxxxxx  tdi 0, r0, 0xa
 {                  "tdlgti GPR , NUM",{0x08200000,0x001FFFFF}}, // 00001000001xxxxxxxxxxxxxxxxxxxxx  tdlgti r0, 0
 {                  "tdllti GPR , NUM",{0x08400000,0x001FFFFF}}, // 00001000010xxxxxxxxxxxxxxxxxxxxx  tdllti r0, 0
@@ -2932,12 +2932,13 @@ int assemble_single(string src, uint32_t addr, uint8_t *result, string& err,
 
 	MYLOG("src:%s has signature:%s\n", src.c_str(), sig_src.c_str());
 
-	if(lookup.find(sig_src) == lookup.end()) {
+	auto it = lookup.find(sig_src);
+	if(it == lookup.end()) {
 		err = "invalid syntax in " + sig_src;
 		return -1;
 	}
 
-	auto info = lookup[sig_src];
+	auto info = it->second;
 	uint32_t vary_mask = info.mask;
 
 	/* for relative branches, shift the target address to 0 */