Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

libc/nexgen32e/bsr.h

@@ -3,26 +3,25 @@
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
 
-unsigned bsr(unsigned) libcesque pureconst;
-unsigned bsrl(unsigned long) libcesque pureconst;
-unsigned bsrmax(uintmax_t) libcesque pureconst;
+/*
+ * BIT SCANNING 101
+ *                          ctz(𝑥)         31^clz(𝑥)   clz(𝑥)
+ *      uint32 𝑥   bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)   bsr(𝑥) lzcnt(𝑥)
+ *    0x00000000      wut       32        0      wut       32
+ *    0x00000001        0        0        1        0       31
+ *    0x80000001        0        0        1       31        0
+ *    0x80000000       31       31       32       31        0
+ *    0x00000010        4        4        5        4       27
+ *    0x08000010        4        4        5       27        4
+ *    0x08000000       27       27       28       27        4
+ *    0xffffffff        0        0        1       31        0
+ */
 
-#ifdef __GNUC__
-#define bsr(X)                                    \
-  ({                                              \
-    unsigned Word;                                \
-    asm("bsrl\t%1,%0" : "=r,r"(Word) : "r,m"(X)); \
-    Word;                                         \
-  })
-#define bsrl(X)                                   \
-  ({                                              \
-    unsigned Res;                                 \
-    unsigned long Word;                           \
-    asm("bsrq\t%1,%0" : "=r,r"(Word) : "r,m"(X)); \
-    Res = Word;                                   \
-    Res;                                          \
-  })
-#endif
+#define bsr(u)   ((sizeof(unsigned) * 8 - 1) ^ __builtin_clz(u))
+#define bsrl(u)  ((sizeof(unsigned long) * 8 - 1) ^ __builtin_clzl(u))
+#define bsrll(u) ((sizeof(unsigned long long) * 8 - 1) ^ __builtin_clzll(u))
+
+unsigned bsrmax(uintmax_t);
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */