Add Minimal Std implementation for UEFI

Implemented modules:
1. alloc
2. os_str
3. env
4. math

Tracking Issue: https://github.com/rust-lang/rust/issues/100499
API Change Proposal: https://github.com/rust-lang/libs-team/issues/87

This was originally part of https://github.com/rust-lang/rust/pull/100316. Since
that PR was becoming too unwieldy and cluttered, and with suggestion
from @dvdhrm, I have extracted a minimal std implementation to this PR.

Signed-off-by: Ayush Singh <ayushsingh1325@gmail.com>