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

third_party/dtoa/dtoa.c

@@ -1,3 +1,4 @@
+#include "libc/bits/pushpop.h"
 #include "libc/calls/calls.h"
 #include "libc/limits.h"
 #include "libc/math.h"
@@ -294,8 +295,7 @@ extern void *REALLOC(void *, size_t);
 #define PRIVATE_MEM 2304
 #endif
 #define PRIVATE_mem ((PRIVATE_MEM + sizeof(double) - 1) / sizeof(double))
-static double private_mem[PRIVATE_mem], *pmem_next;
-INITIALIZER(300, _init_dtoa_pmem, { pmem_next = private_mem; });
+static double private_mem[PRIVATE_mem], *pmem_next = private_mem;
 #endif
 
 #undef IEEE_Arith
@@ -398,11 +398,10 @@ Exactly one of IEEE_8087, IEEE_MC68k, VAX, or IBM should be defined.
 #ifdef SET_INEXACT
 #define dtoa_divmax 27
 #else
-int dtoa_divmax; /* Permit experimenting: on some systems, 64-bit integer */
-                 /* division is slow enough that we may sometimes want to */
-                 /* avoid using it.   We assume (but do not check) that   */
-                 /* dtoa_divmax <= 27.*/
-INITIALIZER(300, _init_dtoa_divmax, { dtoa_divmax = 2; });
+int dtoa_divmax = 2; /* Permit experimenting: on some systems, 64-bit integer */
+                     /* division is slow enough that we may sometimes want to */
+                     /* avoid using it.   We assume (but do not check) that   */
+                     /* dtoa_divmax <= 27.*/
 #endif
 
 typedef struct BF96 { /* Normalized 96-bit software floating point numbers */

third_party/dtoa/dtoa.mk

@@ -46,7 +46,6 @@ $(THIRD_PARTY_DTOA_A).pkg:				\
 
 $(THIRD_PARTY_DTOA_A_OBJS):				\
 	OVERRIDE_CFLAGS +=				\
-		-ansi					\
 		$(OLD_CODE)				\
 		$(IEEE_MATH)				\
 		-ffunction-sections			\

third_party/dtoa/strtod.c

@@ -85,7 +85,6 @@ enum
 	S7			/* _+#.#e+#	#S7 */
 };
 
-static	int	xcmp(char*, char*);
 static	int	fpcmp(char*, ulong*);
 static	void	frnorm(ulong*);
 static	void	divascii(char*, int*, int*, int*);