[profile] Avoid allocating a page on the stack, NFC

When writing out a profile, avoid allocating a page on the stack for the
purpose of writing out zeroes, as some embedded environments do not have
enough stack space to accomodate this.

Instead, use a small, fixed-size zero buffer that can be written
repeatedly.

For a synthetic file with >100,000 functions, I did not measure a
significant difference in profile write times. We are removing a
page-length zero-fill `memset()` in favor of several smaller buffered
`fwrite()` calls: in practice, I am not sure there is much of a
difference. The performance impact is only expected to affect the
continuous sync mode (%c) -- zero padding is less than 8 bytes in all
other cases.

rdar://57810014

Differential Revision: https://reviews.llvm.org/D71323

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