Make improvements

This change progresses our AARCH64 support:

- The AARCH64 build and tests are now passing
- Add 128-bit floating-point support to printf()
- Fix clone() so it initializes cosmo's x28 TLS register
- Fix TLS memory layout issue with aarch64 _Alignas vars
- Revamp microbenchmarking tools so they work on aarch64
- Make some subtle improvements to aarch64 crash reporting
- Make kisdangerous() memory checks more accurate on aarch64
- Remove sys_open() since it's not available on Linux AARCH64

This change makes general improvements to Cosmo and Redbean:

- Introduce GetHostIsa() function in Redbean
- You can now feature check using pledge(0, 0)
- You can now feature check using unveil("",0)
- Refactor some more x86-specific asm comments
- Refactor and write docs for some libm functions
- Make the mmap() API behave more similar to Linux
- Fix WIFSIGNALED() which wrongly returned true for zero
- Rename some obscure cosmo keywords from noFOO to dontFOO

libc/tinymath/rintf.c

@@ -43,6 +43,12 @@ asm(".include \"libc/disclaimer.inc\"");
 #endif
 static const float_t toint = 1/EPS;
 
+/**
+ * Rounds to integer in current rounding mode.
+ *
+ * The floating-point exception `FE_INEXACT` is raised if the result is
+ * different from the input.
+ */
 float rintf(float x)
 {
 	union {float f; uint32_t i;} u = {x};