Raise DEFAULT_MIN_STACK_SIZE to at least 64KiB

Prevent copy-paste errors from producing new starved-for-resources
threaded platforms by raising `DEFAULT_MIN_STACK_SIZE` from 4096 bytes
to at least 64KiB.

Two platforms "affected" by this have no actual threads:
- UEFI
- "unsupported"

Platforms that this actually affects:
- wasm32-wasi with "atomics" enabled
- wasm32-wasi-p1-threads

Two exceptions:
- SGX: a "secure code execution" platform, stays at 4096B
- TEEOS: also a "secure code execution" platform, stays at 8192B

I believe either of these may have sufficiently "interesting" semantics
around threads, or significant external library support. Either would
mean making any choices here for them is suspect.

library/std/src/sys/pal/uefi/thread.rs

@@ -7,7 +7,7 @@
 
 pub struct Thread(!);
 
-pub const DEFAULT_MIN_STACK_SIZE: usize = 4096;
+pub const DEFAULT_MIN_STACK_SIZE: usize = 64 * 1024;
 
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements

library/std/src/sys/pal/unsupported/thread.rs

@@ -6,7 +6,7 @@
 
 pub struct Thread(!);
 
-pub const DEFAULT_MIN_STACK_SIZE: usize = 4096;
+pub const DEFAULT_MIN_STACK_SIZE: usize = 64 * 1024;
 
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements

library/std/src/sys/pal/wasi/thread.rs

@@ -66,7 +66,7 @@ fn drop(&mut self) {
     }
 }
 
-pub const DEFAULT_MIN_STACK_SIZE: usize = 4096;
+pub const DEFAULT_MIN_STACK_SIZE: usize = 64 * 1024;
 
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements

library/std/src/sys/pal/wasm/atomics/thread.rs

@@ -6,7 +6,7 @@
 
 pub struct Thread(!);
 
-pub const DEFAULT_MIN_STACK_SIZE: usize = 4096;
+pub const DEFAULT_MIN_STACK_SIZE: usize = 64 * 1024;
 
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements