Improve signal handling and math

- Polyfill ucontext_t on FreeBSD/OpenBSD/NetBSD
- Add tests confirming signals can edit CPU state
- Work towards supporting ZIP filesystem on bare metal
- Add more tinymath unit tests for POSIX conformance
- Add X87 and SSE status flags to crash report
- Fix some bugs in blinkenlights
- Fix llvm build breakage

libc/tinymath/fmodl.S

@@ -18,8 +18,14 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/runtime/pc.internal.h"
 #include "libc/macros.h"
-.source	__FILE__
 
+//	fmod [sic] does (𝑥 rem 𝑦) w/ round()-style rounding.
+//
+//	@param	𝑥 is an 80-bit long double passed on stack in 16-bytes
+//	@param	𝑦 is the power, also pushed on stack, in reverse order
+//	@return	remainder ∈ (-|𝑦|,|𝑦|) in %st
+//	@define	𝑥-truncl(𝑥/𝑦)*𝑦
+//	@see	emod()
 fmodl:	push	%rbp
 	mov	%rsp,%rbp
 	.profilable
@@ -32,4 +38,7 @@ fmodl:	push	%rbp
 	fstp	%st(1)
 	pop	%rbp
 	ret
+1:	int3
+	pop	%rbp
+	ret
 	.endfn	fmodl,globl