Switch to libc wrapper instead of raw syscall for `gettid()` and `execveat()`

[SteveLauC]

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gettid() was initially added in #293, at that time, we didn't have a libc wrapper so we had to involve the syscall() directly. The wrapper in glibc was added in 2.30, and the libc crate has exposed this interface since this PR, so we probably should switch to the libc wrapper and get rid of the raw syscall.

My concern, on Linux/glibc, it is available since 2.30, but the current supported glibc version is 2.17, so I am afraid we will have conpatibility issue just like this one #2146

[SteveLauC]

execveat() was added in #800, still no libc wrapper at that time, but for now, from its man page, glibc seems to have a wrapper for it since 2.34, the libc crate has not exposed it yet

Update: musl, uClibc and bionic(Android) don't have a wrapper for execveat(), I will add a binding for glibc to the libc crate

[SteveLauC]

I will add a binding for glibc to the libc crate

PR filed

[asomers]

Nice research.