fbvector unaligned memory allocations lead to sigsegv

fbvector uses `malloc` instead of allocator methods for memory allocations if allocator type is `std::allocator` ([code](https://github.com/facebook/folly/blob/main/folly/FBVector.h#L126))

`malloc` usually aligns output data on 16 bytes boundary, but compiler may generate AVX instruction for initialization, such as `vmovaps` that requires alignment on a 64/128 byte boundary

Problem code example:
```c++
#include
#include
#include

struct alignas(128) aligned_struct {
aligned_struct() {
std::memset(padd, 0, sizeof(padd));
}

char padd[128];
};

int main() {
folly::fbvector> folly_vec(5); // segfault with -march option with AVX instructions, e.g. -march=icelake-server
std::vector> std_vec(5); // works, at least on C++20

return 0;
}
```

The main problem is that memory is not allocated using specified allocator methods under the hood that is not obvious

Start at folly/FBVector.h around line 126 and reproduce the aligned_struct example with an AVX-enabled compiler. Trace the std::allocator allocation path, then verify that fbvector honors the allocator’s alignment requirements and no longer segfaults; the payload names no test file, so identify the relevant FBVector test before changing behavior.

Written by the indexing model from the issue text.